An OFDM scheme uses a plurality of narrowband subcarriers and modulates the narrowband subcarriers to transmit a signal. Therefore, the OFDM scheme, whose frequency spectrum is close to a square shape as a whole, obtains high frequency utilization efficiency. In addition, the OFDM scheme is robust against a delayed wave because a symbol length is long as compared to a single carrier scheme. Further, the OFDM scheme can be robust within a multipath environment by adding a guard interval (GI).
In addition, in an orthogonal frequency division multiple access (OFDMA) scheme using the OFDM scheme, it is possible to simultaneously transmit information to a plurality of communication partners by allocating subcarriers to a plurality of information transmission destinations.
In an integrated services digital broadcasting-terrestrial (ISDB-T) scheme, which is a terrestrial digital television broadcasting scheme in Japan, a band segmented transmission-OFDM (BST-OFDM) scheme is adopted. In the BST-OFDM scheme, it is possible to select more appropriate settings (a radio modulation scheme, transmission power, and the like) according to a purpose, by dividing subcarriers of the OFDM scheme into groups called segment and allocating the segments to each purpose.
Further, in a long term evolution (LTE) system designed by Third Generation Partnership Project (3GPP), the OFDMA scheme is adopted for downlink from a base station to a mobile station and a single carrier-frequency division multiple access (SC-FDMA) scheme based on discrete Fourier transform (DFT)-spread OFDM is adopted for uplink from the mobile station to the base station.
In the SC-FDMA scheme based on the DFT-spread OFDM, a time waveform of a symbol sequence to be transmitted, is subjected to a DFT and then designated as an input of the OFDM scheme. Frequency components of the symbol-sequence time waveform to be transmitted, which are DFT output input to OFDM, may be allocated to subcarriers indicated in advance from the base station in subcarrier mapping. The number of allocated subcarriers is set according to demand, and not all subcarriers are constantly allocated.
The OFDM scheme is multicarrier transmission comprised by many subcarriers. An OFDM signal, which is an output of the OFDM scheme, has high peak power when peak values of subcarriers overlap. An example of a baseband OFDM signal waveform is illustrated in FIG. 6. When the OFDM signal as described above is input to a PA having non-linearity characteristics, undesirable characteristic degradation, such as degradation of transmission characteristics or the increase of out-of-band radiation, is caused. Therefore, a PA having low non-linear distortion characteristics is needed for an OFDM modulated wave transmitter apparatus.
Generally, in case of a comparison under the same output power, an amplifier having higher saturation output power characteristics has lower non-linear distortion characteristics. However, the amplifier as described above is not preferable in terms of the power efficiency because power consumption is high during a small signal of which non-linear distortion is not problematic. Consequently, it is necessary to improve the non-linear distortion characteristics without degrading the power efficiency.
To reduce the non-linear distortion in OFDM, a scheme of improving linearity during peak power generation is proposed, for example, in Patent Document 1. In the scheme of improving the linearity during the peak power generation, the linearity is improved by temporarily applying a high voltage or a large current to the PA when high peak power is generated. This scheme can improve transmission characteristics and out-of-band radiation characteristics of an OFDM signal in a range not exceeding a maximum rating of a component of a high-power amplifier and a range that does not adversely affect reliability.